The present invention generally relates to RAKE receivers and particularly relates to the placement of RAKE fingers in a RAKE receiver.
RAKE receivers represent a well-known approach to multipath reception, particularly in Direct Sequence Code Division Multiple Access (DS-CDMA) wireless communication systems. With multipath, a transmitted signal follows multiple propagation paths and the intended receiver thus receives a “composite” signal that may include multiple “versions” of the transmitted signal, with each version generally suffering from different path delay, phase, and attenuation effects. The different versions of the received signal, which may be referred to as “signal images,” thus arrive at the receiver slightly ahead of or behind the other images. The maximum delay spread between signal images, i.e., the “dispersion,” depends on, among other things, the signal bandwidth and the differing characteristics of the signal propagation paths.
Normally, a RAKE receiver includes a plurality of “fingers,” wherein each finger operates as a despreading circuit, i.e., a correlator that is configured to despread a signal image at a configurable relative delay. Nominally, the RAKE receiver aligns its available RAKE fingers to the strongest signal images, such that each selected signal image is despread and then combined in subsequent processing. Combining the multiple signal images in this manner generally provides improved signal to noise ratios at the receiver.
Supporting these operations, the RAKE receiver includes, or otherwise cooperates with, a searcher that identifies signal peaks in the received signal across a defined search window. Nominally, each signal peak corresponds to a different signal image, but because of “smearing” between closely spaced signal images, one or more of the delay peaks may actually represent multiple signal images. The searcher identifies signal peaks using a searcher delay grid, and the RAKE receiver time aligns one or more of its fingers to those searcher-identified delay peaks.